1. Field of the Invention:
The present invention relates generally to nuclear reactor facilities, and more particularly to a system incorporated within the nuclear reactor refueling machine which is capable of remotely mechanically latching fuel assemblies to, and remotely mechanically unlatching the assemblies from, the lower core support plate of the reactor internals, in addition to being capable of conventionally gripping the core fuel assemblies as well as raising, lowering, or transporting the assemblies during the performance of refueling operations.
2. Description of the Prior Art:
As is well known in the nuclear reactor art, fuel, conventionally in the form of pellets, is inserted within suitable cladding material, and the composite assemblage of the fuel pellets and the cladding material or casings serve to define or form the nuclear reactor fuel rods. In turn, a predetermined number of fuel rods, assembled or secured together by means of bands called grid straps, serve to form or define a fuel element or fuel assembly, and a predetermined number of fuel elements or fuel assemblies serve to define or form the nuclear reactor core. As a result of the normal operation of the nuclear reactor facility, the nuclear fuel within the core fuel assemblies naturally becomes depleted, and consequently, the reactor core fuel assemblies must be periodically replaced and refueled. This is achieved by means of conventional refueling operations and techniques.
In particular, the fuel within the reactor core fuel assemblies is depleted over a predetermined period of time and at a predetermined consumption rate such that once an initially new reactor facility has attained its steady state fuel consumption activity or operation through means of having undergone, for example, an initial two-year stabilization period of operation, each fuel assembly utilized within the reactor core will have a service life of three years. In lieu of refueling the entire reactor core once every three years by replacing all of the core fuel assemblies with newly fresh fuel assemblies, maintenance requirements and economic considerations have dictated that the reactor core be refueled once per year, during which period the reactor facility is of course shut down. In order to achieve or accommodate such requisite refueling operations, the reactor core is sectionalized, and the fuel supply relatively staggered between the core sections or stages. Specifically, the reactor core fuel assemblies are effectively arranged within three groups, sections, or stages, including a first, central circular section, a second intermediate annular section disposed about the first central section, and a third outermost annular section disposed about the second intermediate annular section. In addition, as a result of the aforenoted initial two-year stabilization period of operation, at the end of any subsequent one-year period of operation, the nuclear fuel disposed within the fuel assemblies of the innermost or first central section of the reactor core, which fuel assemblies have been disposed within the reactor core for an operational period of three years, will have been substantially entirely depleted. Similarly, the nuclear fuel disposed within the fuel assemblies of the second intermediate or middle section of the reactor core, which fuel assemblies have been in operational service within the reactor core for a period of only two years, will be sufficient so as to permit such fuel assemblies to provide service within the reactor core for an additional period of one year. In a like manner, the nuclear fuel disposed within the fuel assemblies of the third outermost section of the reactor core, which fuel assemblies have been in operational service within the reactor core for a period of only one year, will be sufficient so as to permit such fuel assemblies to provide service within the reactor core for an additional period of two years. In accordance with conventional refueling techniques, then, the fuel assemblies from the innermost or central section of the core are removed from the reactor core for actual refueling with fresh or new fuel, while the fuel assemblies disposed within the intermediate or middle section of the core are transferred to the first central section of the core. Continuing further, the fuel assemblies disposed within the outermost third section of the core are transferred to the second intermediate or middle section of the core, while entirely new or fresh fuel assemblies are inserted into the outermost third section of the core, thereby completing the refueling operation of the reactor facility.
The fuel assemblies must of course be fixedly secured to the upper and lower core support plates of the reactor so as to be securely stabilized under the influence, for example, of the hydraulic forces attendant the coolant which is circulated throughout the reactor core during operation of the facility. However, in view of the foregoing requirements of the refueling operations, the assemblies must likewise be capable of being readily disconnected from the upper and lower core support plates so that the refueling operations may in fact be performed. In order to accomplish the foregoing attachment and stabilization goals, the fuel assemblies have been conventionally provided with suitable spring packs or assemblies. As the reactor facility technology has become more sophisticated, however, including, for example, an increase in coolant circulation requirements, the intensified hydraulic flow rates and forces attendant such increased coolant circulation requirements has correspondingly necessitated the need for increasingly more complex and costly spring packs or assemblies. Still further, while the fuel assemblies and core internals elements or structures of older generation reactor facilities were all fabricated from stainless steel, the fuel assemblies of the newer generation reactor facilities have been fabricated from zirconium in view of the lower neutron absorption characteristics of such material. As a result of this difference in materials, and the corresponding difference in the coefficients of thermal expansion characteristic of such materials, conventional fuel assemblies exhibit considerable thermal growth properties relative to the upper and lower core support plates. Consequently, the aforenoted spring assemblies or spring packs had to be modified further so as to include operational features which would permit the spring assemblies or spring packs to operationally accommodate such thermal growth properties, in addition to preserving the aforenoted attachment and stabilization requirements, of the fuel assemblies. These structural modifications have of course increased the complexity and cost factors of the fuel assembly spring packs still further.
In view of the foregoing, recent developments in nuclear reactor fuel assembly technology have resulted in the fabrication of fuel assemblies which can be mechanically connected, for example, to the lower core support plate of the reactor internals, and in this manner, the aforenoted complex and costly fuel assembly spring packs have been able to be eliminated. A need, however, exists for technology which will permit and facilitate the remote latching and unlatching of such mechanically-connectable fuel assemblies to and from the lower core support plate of the reactor internals.
Accordingly, it is an object of the present invention to provide a new and improved nuclear reactor refueling machine.
Another object of the present invention is to provide a new and improved nuclear reactor refueling machine which will satisfy the aforenoted need for technological apparatus which is capable of remotely latching and unlatching mechanically-connectable fuel assemblies to and from the lower core support plate of the reactor internals.
Yet another object of the present invention is to provide a new and improved nuclear reactor refueling machine which, in addition to its capability of remotely latching and unlatching mechanically-connectable fuel assemblies to and from the lower core support plate of the reactor internals, is also capable of gripping the fuel assemblies, and raising, transporting and lowering the fuel assemblies during the performance of a refueling operation within a nuclear reactor facility.
Still another object of the present invention is to provide a new and improved nuclear reactor refueling machine wherein all of the apparatus or equipment which is required for the performance of the various functions or procedures attendant a refueling operation, such as, for example, the gripping of a particular fuel assembly within the reactor core, the remote unlatching of the same from the lower core support plate of the reactor internals, the raising of the fuel assembly relative to the reator core, the transporting of the fuel assembly to a new core location as may be desired, the lowering of the fuel assembly into the new core location, the remote latching of the newly deposited fuel assembly to the lower core support plate, and the disconnection of the gripping mechanisms from the fuel assembly top nozzle, may be incorporated, in a substantially co-axial manner, within a single mast of the refueling machine.